This paper presents a theoretical and experimental study on the possibility of combustion similarity in differently sized diesel engines. Combustion similarity means that the flow pattern and flame distribution develop similarly in differently sized engines. The study contributes to an understanding and correlating of data which are presently limited to specific engine designs.The theoretical consideration shows the possibility of combustion similarity, and the similarity conditions were identified. To verify the theory, a comparison of experimental data from real engines was performed; and a comparison of results of a three dimensional computer simulation for different engine sizes was also attempted. The results showed good agreement with the theoretical predictions.THE PURPOSE of this research is to determine the possibility of the existence of combustion similarity in differently sized diesel engines, and to propose conditions for realizing model experiments.Diesel engine combustion has been widely investigated, but there is no theory of engine size effects. Engine performance in large engines is estimated from experience, without the benefit of model experiments to perform fine optimization. The establishment of a theory of size effects could provide a method for correlation of the vast amount of independent data that is available, and offer significant opportunity for diesel engine research and design.It should be noted that this study does not try to simply compare large and small engines, which are generally designed with different concepts and have different combustion chamber configurations. The study investigates conditions necessary for the establishment of combustion similarity, even though these conditions might not be practical. An ultimate objective of the research is to determine the reasons why combustion chamber configurations are different for large and small size engines.The first study of scale effects in engines was reported by Taylor(1) in 1966. His considerations were based on the fact that the mean effective pressure and mean piston speed are roughly independent of the cylinder size. This relation is very basic, but does not enable an estimation of combustion similarity.The symposium on “Size effect and similarity theory on diesel engine combustion”, Tokyo, March 1982, was of direct relevance to combustion similarity in diesel engines (2). The symposium covered similarities in fuel spray, air motion, heat transfer, combustion, lubrication, and engine performance. The main conclusion of the symposium was that the establishment of a similarity theory would be impossible. However, the evaluation of the data presented at the symposium may be expanded with the theory presented here, because the discussion then was based on simple comparisons of experimental data without non-dimensional treatment.There is a report on similarity in boilers (3). Here theoretical considerations of flow patterns and combustion rates for burner combustion were presented. In the design of large marine engines, Kawasaki Heavy Industries has produced a number of engines with similar geometry, and has succeeded in obtaining similar combustion patterns, as well as vibration and mechanical stresses (4,5).With this background, the authors first presented a theory on diesel combustion similarity to Transactions of the Japan Society of Mechanical Engineers in 1988 (6). The theory showed the fundamental equations in appropriate non-dimensional form were similar for differently sized diesel engines. The equations included continuity, momentum, energy, chemical reaction, continuity of species, and state equations.An English translation of the paper has been published in the International Journal of the Japan Society of Mechanical Engineers in 1990 (7). Following this report, the authors presented experimental results partially validating the theoretical predictions at the International Symposium COMODIA in 1990 in Kyoto (8). The paper discussed similarity in the development of fuel jet in a model apparatus, and it compared the thermal efficiency, heat release rate, and emissions of real engines varying from 260 to 400mm in bore diameter.The authors have also presented a theoretical study on the relationship between optimum swirl and fuel injection systems (9). This study predicted air entrainment changes in fuel sprays for a variety of engine conditions. It did not evaluate similarity between engines, but contains equations applicable for differently sized engines.The present report discusses and modifies the theoretical consideration provided in the previous reports (6,7,8) with more precise equations. The paper also discusses similarity in emissions, and factors which may cause deterioration in similarity. Additionally it shows the results of computer simulations of the possibility of similarity. The report also briefly describes the main results of the experiments presented in the previous work.